Module 5: Chapter 19 - Equilibrium

Question 1

What are the units for Kc?

Answer 1

The units of Kc depends on the number of concentration terms on the top and bottom of the equilibrium constant term

Question 2

How do you determine the units of Kc?

Answer 2

Substitute units into the expression for Kc and determine the units for Kc

Question 3

What are the 2 types of dynamic equilibria?

Answer 3

• Homogeneous Equilibrium
• Heterogeneous equilibrium

Question 4

What is homogeneous equilibrium?

Answer 4

A homogeneous equilibrium contains equilibrium species that all have the same state or phase. (i.e all gases or all liquids)

Question 5

What is hetereogeneous equilibrium?

Answer 5

A heterogeneous equilibrium contains equilibrium species that have different states or phases (i.e some gases and some liquids or solids)

Question 6

How do you calcualte Kc for homogeneous equilibria?

Answer 6

The Kc expression contains concentrations of all species

Question 7

How do you calculate Kc for heterogeneous equilibria?

Answer 7

As the concentrations of liquids and solids are essencitally constant, they are omitted from the Kc expression, Kc only includes species that are (g) or (aq)

Question 8

What is Kp?

Answer 8

The equilibrium constant in terms of partial pressures

Question 9

What are partial pressures?

Answer 9

In a gas mixture, the partial pressure of a gas is the contribution that the gas makes towards the total pressure. This would be the pressure it would exert if it alone occupied the whole container. The sum of all the partial pressures is the total pressure

Question 10

What is the equation for partial pressure?

Answer 10

Partial Pressure (A) = mole fraction of A x Total Pressure

Question 11

What is the mole fraction of a gas?

Answer 11

The proportion of moles of a certain gas to the total number of moles of gas in the mixture. This is the same as the proportion by volume of a has to the total volume of gases in the mixture as one mole of any gas under the same conditions of temperature and pressure occupies the same volume of space.

The sum of mole fractions of all gases in a mixture is equal to one

Question 12

What is the equation for mole fraction?

Answer 12

Mole fraction (A) = Number of moles of A / Total number of moles of gas

Question 13

How do you find Kp?

Answer 13

It is the same equation as Kc but is calculated with partial pressures rather than concentration

Question 14

What is the equation for Kp?

Answer 14

For the equation aA(g) + bB(g) ⇌ cC(g) + dD(g)

Kp = p(C)ᶜp(D)ᵈ/p(A)ᵃp(B)ᵇ

p(X) is the partial pressure of X

Question 16

What is important to consider when calculating Kp?

Answer 16

ONLY gases are included, solids, liquids, and solutions are not included

Question 17

What is the only factor which can affect the equilibrium constant?

Answer 17

Temperature, at a set temperature K is constant and any other changes in concentration, pressure, or presence of a catalyst will not effect K. However, if the temperature is changed, K will change

Question 18

What is the effect on the equilibrium constant if the temperature of an exothermic reaction is increased?

Answer 18

If the forward reaction is exothermic:
* The position of equilibrium moves to the left
* The equilibrium constant decreases with increasing temperature
* Raising the temperature decreases the equilibrium yield of products

Question 19

What is the effect on the equilibrium constant if the temperature of an endothermic reaction is increased?

Answer 19

If the forward reaction is endothermic:
* The position of equilibrium moves to the right
* The equilibrium constant increases with increasing temperature
* Raising the temperature increases the equilibrium yield of products

Question 20

How does changing the temperature change the position of equilibrium with respect to Kc?

Answer 20

Changing the temperature will cause the reaction to move in either the endothermic or the exothermic direction in order to conteract the change. However, the temperature cannot directly effect the concentration of the reactants, therefore it causes Kc to change in order to bring about this change in the position of equilibrium. Only temperature can change Kc

Question 21

Explain how changes in concentration cause a shift in equilibrium

Answer 21

Changes in concentration do not affect the equilibrium constant and this is exactly what causes the shift in equilibrium point. If the concentration of reactants is increased the ratio of “[products]/[reactants]” will decrease and is now less than Kc. Therefore the system is no longer in equilibrium. As a results the concentrations must change to return to the ratio of “[products]/[reactants]” that matches Kc. Therefore, the concentration of products must icnrease and the concentration of reactants must decrease

Question 22

Explain how changes in pressure cause a shift in equilibrium

Answer 22

Changes in pressure do not affect the equilibrium constant and this is exactly what causes the shift in equilibrium point. If you double the pressure of the system, the partial pressures of each gas will double aswell. However, as the ratio involves partial pressures raised to different powers (due to different molar ratios in the chemical equation) it will change the ratio of “p(products)/p(reactants)” to be different to that of Kp. This means the system is no longer in equilibrium. The partial pressures must then change to restore the ratio of “p(products)/p(reactants)” to be the same as Kp. This also explains why the position of equilibrium does not change if there are equal number of moles on either side of the equation, as the ratio of “p(products)/p(reactants)” would not change with pressure.

Question 23

How does a catalyst affect equilibrium constants?

Answer 23

A catalyst does not effect the equilibrium constant as they effect the rate of a chemical reaction but not the position of equilibrium. Catalysts speed up both the forward and reverse reactions by the same factor. Equilibrium is reached quicker by the equilibrium position is not changed

Question 24

How do you approach Equilibrium constant calculations?

Answer 24

Use ICE:
1. Write the Initial number of moles of each species
2. Write the Change in the number of moles of each species - this change is directly proportional to the molar ratio of each species in the chemical equation
3. Write the number of moles of each species at Equilibrium
4. Calculate concentrations/partial pressures
5. Calculate Kc or Kp

Question 25

Answer 25

Kc = 0.184 No Units

Question 26

Answer 26

12.5 dm³ mol⁻¹

Question 27

Answer 27

1.75g